ABSTRACT

   Iron oxide-copper-gold mineralisation within the Turkish Tethyan collage defines a group of diverse, epigenetic Cu-Au deposits. As a part of the Tethyan collage, Turkey hosts several IOCG deposits formed in post-collisional, late orogenic, extensional settings, related to subduction of NeoTethyan oceanic crust beneath the Eurasian plate during late Cretaceous to Miocene time. Research on these deposits in Turkey has revealed that some of the iron-oxide mineralisation occurs in metasomatised magmatic rocks that underwent pervasive sodic-calcic and superimposed potassic alteration. These deposits exhibit comparable alteration styles, represented by pre-ore sodic-calcic alteration, overprinted by relatively late syn-ore potassic alteration and magnetite mineralisation, and by post-ore sericitisation and sulphide mineral assemblages. Subsequent alteration, consisting essentially of oxidised sulphides and iron, has in turn been superimposed on the preceding styles. Magnetite is commonly associated with potassic zones, while copper-gold mineralisation accompanies late stage sericitic alteration and carbonatised rocks, mainly along structural discontinuities. The host rocks and alteration are confined to crustal scale regional strike-slip and normal faults.
   The Ar-Ar, U-Pb and K-Ar geochronology of minerals from the alteration and magmatic rocks, shows that the interval over which hydrothermal alteration and mineralisation occurred (~74 to 22 Ma), overlaps with the period of calc-alkaline to alkaline magmatism in both the central Anatolian extensional post-collisional, and western Anatolian extensional provinces. The spatial association between the alteration and the magmatic rocks, suggests that the crystallisation and cooling of the Late Cretaceous post-collisional alkaline to calc-alkaline magmatism of central-eastern Anatolia, and the Oligocene-Miocene calc-alkaline magmatism in western Anatolia, contributed to the hydrothermal systems over the entire duration of the metallogenic history of the IOCG provinces. The styles of hydrothermal alteration, mineralogy, paragenesis, stable and radiogenic isotope systematics, and the geochronological data presented here, favour a model in which the fluids responsible for the alteration are largely magmatic. This is further supported by stable and radiogenic isotope data on alteration and associated magmatic rocks.
   Based on geochronological evidence and regional compilations, periods favourable for IOCG mineralisation are 74 to 69 Ma in central Anatolia, and 23 to18 Ma in western Anatolia. These intervals are coincident with crustal-scale extension due to post-collisional roll-back of the subducting slab along the Bitlis-Zagros subduction zone (central-eastern Anatolia) and the Aegean subduction complex (western Anatolia). From a Turkish perspective, a demonstration that post-collisional and extensional-intracontinental magmatic settings are fertile for IOCG deposits is critical, as it significantly furthers analogies with both the Canadian 1.9 to 1.5 Ga continental arc settings, and the active, extensional plate margins of the eastern Pacific.

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